Multi-moded acoustic wave oscillator

ABSTRACT

A multi-moded acoustic wave oscillator comprises an amplifier and feedback loop incorporating an acoustic wave delay line which may be a surface acoustic wave delay or bulk acoustic delay. Frequency of oscillation is changed in steps by taking different phased outputs from the delay line. This phase difference is achieved either by spacing surface acoustic wave transducers different distances on a substrate from an input transducer or by phase shift circuitry.

This invention relates to acoustic wave oscillators. Such oscillatorscan use surface acoustic waves (SAW) or bulk acoustic waves (BAW).

In a typical SAW oscillator a SAW delay line is used as the positivefeedback element of an amplifier. A SAW delay line may comprise a quartzsubstrate carrying an input and an output transducer on one flatsurface. Usually such SAW delay lines permit oscillation at a pluralityof frequencies i.e. the various modes permitted by the substrate betweenthe two transducers. A disadvantage of this oscillator is the control ofthe precise oscillation frequency. One arrangement for controlling thefrequency of oscillation uses a high Q resonant circuit within thefeedback loop but external to the delay line.

A quite different approach uses the frequency response of thetransducers and intertransducer spacing to ensure oscillation at onefrequency only; such an arrangement is described in U.K. patentapplication 7880/73, U.S. patent application Ser. No. 442,624 (now U.S.Pat. No. 3,950,713).

Neither of these prior art arrangements allow a quick and precise changeof frequency of oscillation from one mode to another.

It is therefore an object of this invention to provide an oscillatorcapable of oscillating at a plurality of preset and controllablefrequency modes.

According to this invention a multi-moded acoustic wave oscillatorcomprises an amplifier and a feedback loop incorporating an acousticwave delay line, the delay line comprising a substrate able to supportacoustic waves, an input transducer for launching acoustic waves along atrack in the substrate, a plurality of output transducers each arrangedon the track, phase changing means for supplying a phase differencebetween inputs to the amplifier from the different output transducers,and means interconnecting the output transducer for allowing each outputtransducer to be connected in turn to the amplifier, the arrangementbeing such that the frequency of oscillation is changed in preset stepsas the output transducers are connected in turn to the amplifier.

The output transducers may be spaced different distances from the inputtransducers. Alternatively the output transducers may be equispaced fromthe input transducer and phase change circuits arranged between theoutput transducers and amplifiers.

The roles of the input and output transducers may be reversed, i.e. thedelay line may use a plurality of input transducers and a single outputtransducer.

The substrate may be also piezo electric e.g. quartz or lithium niobate,or non-piezoelectric, e.g. glass, with pads of a piezoelectric materialat the transducers.

The delay line may also take other forms e.g. a conventional bulk-wavedelay line transducers at either end, and say two output transducers.

The transducers may have constant finger length and spacing, in whichcase the amplitude response is of the sin x/x type and the modes ofoscillation are equally separated in frequency. However, it is possibleto weight the finger strengths in various ways (c.f. design of SAWbandpass filters) to have, say a flatter top to the main lobe, therebymaking all wanted modes more nearly equally favoured. This may bedesirable in some circumstances.

It is also possible to make the finger spacings within the input and/orthe output transducers vary continously along the length of thetransducer (i.e. perpendicular to the fingers) in such a way that thedelay line(s) becomes dispersive (as used in pulse compression radar).In these circumstances the effective path lengths are a function offrequency and the simple equations given later for the allowedfrequencies of oscillation no longer obtain. The modes are no longerequally spaced in frequency but their values may be calculated readilyfrom the theory of the pulse compression filters which is well-known inthe art.

The invention will now be described by way of example only withreference to the accompanying drawings of which

FIG. 1 is a diagrammatic view of an oscillator having four outputtransducers;

FIG. 2 is a frequency response graph;

FIG. 3 shows a modification to FIG. 1;

FIG. 4 shows an alternative to the device of FIG. 1;

FIG. 5 shows a BAW delay line to replace the SAW delay line of FIG. 3.

FIG. 6 shows a modification of FIG. 4 using dispersive and weightedtransducers.

The oscillator of FIG. 1 comprises a SAW delay line 1 and rotary switch2 in the feedback loop 3 of an amplifier 4. The delay line 1 comprises apiezoelectric substrate 5, e.g. ST cut quartz, which carries an inputtransducer 6 and four output transducers 7a, b, c, d spaced λ₀ /4 apart,measured along a SAW track 8 from the input transducer, λ_(o) beingwavelength at centre frequency f_(o). The output of each outputtransducer 7 is connected to the rotary switch 2 which has four equalresistors R₁, R₂, R₃, R₄ and a central rotatable wiper 9. Output fromthe wiper 9 is to the input of the amplifier 4 whose output is to theinput transducer 6. The four output transducers 7 may have equal lengthfinger electrodes, as shown, or unequal length transducers. For example,if it is intended to operate the device with transducer 7a (the othersbeing used solely to change the frequency) then transducer 7a may havelong electrodes, while smaller electrodes will suffice in the remainingthree output transducers 7b, c, d.

The path length L between the input transducer 6 and the upper outputtransducer 7a is chosen to give the required device stability and/or thefrequency separation of the modes.

When simple transducers of the type shown in FIG. 1 are employed it canbe shown that the number of complete cycles, n, around the loopcomprising the delay line and amplifier is satisfied by the equation

    wL/C + φ = 2πn

where

w = angular frequency of acoustic waves

L = path length

C = acoustic velocity on the substrate

φ = the small electrical phase shift associated with amplifier andtransducers

n = an integer, often called the mode number.

Thus neglecting φ the allowed frequencies of oscillation are given by

    w = 2(Cπ/L), 4Cπ/L, 6Cπ/L etc.

The response of a transducer has the form sin x/x and is determined fromstandard formulae to provide a wide central lobe encompassing thedesired number of modes, see FIG. 2. By weighting the transducers it ispossible to flatten the top of this response to some extent which is adesirable improvement to the simple device.

FIG. 2 shows the frequency response 10 of the input transducer and themodes i.e., harmonic frequencies of oscillation 11a permitted by thepath length L. Note that the modes 11a shown are those for the pathlength L between the input transducer and the upper output transducer7a. Modes 11b, 11c, 11d also occur between those shown for the pathlengths L + λo/4, L + λo/2, and L + 3λo/4 corresponding to the otheroutput transducers 7b, 7c, 7d. Amplifier gain is set, as shown by line12, so that side lobe responses cannot be excited.

In operation the device of FIG. 1 is switched on and, after a shortstabilising period it will oscillate at one of the permitted modes. Ifthe wiper 9 of the rotary switch 2 is gradually rotated anti-clockwiseoutput into the amplifier 4 will be from the four output transducers 7a,b, c, d separately and in turn. Since the output transducers 7 arespaced λ_(o) /4 along the SAW track the phase of signal into theamplifier 4 changes by steps of π/2. One complete revolution of thewiper 9 changes the frequency of oscillaton from one mode 11a (of pathlength L) to the next, decreasing the frequency for anti-clockwiserotation, or increasing the frequency for clockwise rotation.

If the wiper 9 is merely switched by 1/4 turn the device would settledown to oscillate at a frequency between the modes 11a shown in FIG. 2i.e. a mode 7b or 7d corresponding to path length L + λo/4.

As shown in FIG. 1 all the output transducers 8 are on one side of theinput transducer. In a modification transducer the four outputtransducers 7 are arranged two either side the input transducer.

More than four output transducers may be used, with correspondinglysmall steps in path length, arranged on one or both sides of the inputtransducer. In general such an arrangement gives smoother operation andpermits operation over a larger bandwidth.

A typical device has a centre frequency of 60 MHz, with a path length L= 3.15 cms on ST-cut quartz the separation of modes is 0.1 MHz. Theinput transducer has 20 finger pairs and each output transducer has 20finger pairs.

The device of FIG. 1 with its rotary switch 2 is a useful one forillustrating the principle of this invention. A more practical devicewould replace the rotary switch 2 with four PIN diodes suitably biassedin turn. Any other solid state electrically controlled switches such asvoltage-controlled impedance e.g. varactor, or amplifiers can be used inplace of the PIN diodes.

In operation it is not necessary to switch instantly from one outputtransducer to another. It is quite satisfactory (and often useful) toswitch to a combined output at intermediate points, corresponding tointermediate points in the rotary potentiometer of FIG. 1, or to the useof make-before-break mechanical switches. This gives smoother operation,and also permits a further series of intermediate frequencies forexample, midway between the frequencies which would obtain withtransducers 7a or 7b in FIG. 1. FIG. 3 shows a modification of FIG. 1having PIN diodes controlling the output from the delay line. Theoscillator, as before, comprisies a SAW delay line 14 in the feedbackloop 15 of an amplifier 16. This delay line 14 comprises a quartzsubstrate 17 carrying an interdigital input transducer 18 and twointerdigital comb output transducers 19, 20 spaced λ_(o) /4 apartmeasured along a SAW track 21 from the input transducer 18. Both thecombs 19a, b, 20a, b forming each output transducer 19, 20 are connectedto two PIN diodes 22a, 22b, 23a, 23b, 24a, 24b, 25a, 25b the commonpoints of which are connected through 1 k ohm resistors to terminals ofa two pole four way switch 27. One diode in each pair is connected toearth whilst the other is connected to a junction 28 and to theamplifier 16. Positive and negative voltages are supplied to the twopoles of the switch 27 so that when a positive voltage is supplied toone comb of a transducer a negative voltage is supplied to the othercomb thereby connecting one comb to the amplifier and the other toearth. In this manner the combs are connected in turn to the amplifier16 as the switch 27 is rotated.

The two combs of an interdigital transducer are 180° out of phase andtherefore by reversing the earth and live terminals a 180° change inphase can be affected. By this means the transducers 7c and 7d of FIG. 1are effectively replaced using transducers 7a and 7d with theirterminals reversed.

Thus the oscillator of FIG. 3 operates in the same manner as that ofFIG. 1 to change modes, namely output is taken from:- the upper comb 19aof output transducer 19, (with the lower comb 19b earthed) followed bythe upper comb 20a of output transducer 20 (with the lower comb 20bearthed), followed by output from the lower comb 19b of outputtransducer 19 (with the upper comb 19a earthed), followed by output fromthe lower comb 20b of output transducer 20 (with the upper comb 20aearthed).

As shown both output transducer 19, 20 are on one side of the inputtransducer 18; they may also be arranged one on either side of the inputtransducer 18 in which case all the transducers 18, 19, 20 may haveequal finger length.

FIG. 4 shows another form of the invention which uses three outputtransducers. As before it comprises an amplifier 29 with a SAW delayline 30 in the feedback loop 31. The delay line 30 comprises a quartzsubstrate 32 carrying an input transducer 33 and three outputtransducers 34, 35, 36 spaced λ_(o) /3 along a SAW track 37. Output fromthe output transducers 34, 35, 36 is through PIN diodes 38, 39, 40 to ajunction 41 and then to the amplifier 29. These PIN diodes are biassedin turn by application of a suitable voltage through a make before breakthree way switch 42 and 1 k.ohm resistors to connect the outputtransducers 34, 35, 36 in turn to the amplifier 29. The 1 k.ohmresistors limit d.c. current to the PIN diodes and also act as highimpedance blocks to RF signals.

To change the frequency of oscillation the output transducers 34, 35, 36are connected in turn to the amplifier 29, in the same manner as for theoscillator of FIG. 1. One advantage of using three transducers (steppedby λ_(o) /3) is that the amplitude of output from the output transducersis constant whilst output is being switched, even through theintermediate positions where two transducers are connectedsimultaneously.

For devices to operate in the manner described, it is necessary for eachchange in phase to be < 180° (ideally < 120°). Using such criteria, itcan be shown that operating bandwidth of the device can be of the orderof the centre frequency, fo. However for such large bandwidths thespacings of all the intermediate frequencies become unequal, e.g. in theexample with four transducers, the spacing is exactly λ_(o) /4 only atthe centre frequency. This can be a problem if it is desired to use allthe intermediate frequencies, e.g. in an f.m. tuner with equal channelseparations.

One partial solution to this problem is to modify the positions oftransducers 7c and 7d in FIG. 1, so that transducers 7a and 7c areequidistant from the input transducer 6 and 7b and 7d are equidistantfrom the input transducer 6. Additionally the outputs from 7c and 7d aretaken from the lower combs instead of the upper combs shown in FIG. 1.By so reversing the connections to 7c and 7d a phase shift of 180° isintroduced which compensates for the displacement by λ_(o) /2 at thecentre frequency. By this means the frequencies from 7a and 7d haveequal separation at al frequencies, and the same is true of thefrequencies frm 7b and 7c.

Another solution to the above problem is to modify the arrangementsshown in FIGS. 1 and 4 by replacing the non-dispersive transducers shownand use dispersive transducers. This is shown in FIG. 6 which is similarin operation to FIG. 4. The device of FIG. 6 comprises an amplifier 29with a SAW delay line 65 in the feedback loop 31. The delay line 65comprises a quartz substrate 66 carrying a dispersive input transducer67 and three dispersive output transducers 68 69 70 spaced λ_(o) /3along a SAW track 71, each output transducer having varying fingerlength to provide weighting. As in FIG. 4 output from the transducers68, 69, 70 is through PIN diodes 38, 39, 40 to a junction 41 and then tothe amplifier 29. These PIN diodes are biased in turn by application ofa suitable voltage through a make before break three way switch 42 and a1 kohm resistor 72, 73, 74 to connect the output transducers 68, 69, 70,respectively in turn to the amplifier 29. The oscillator of FIG. 6operates in the same way as the oscillator of FIG. 4.

With the dispersive transducers 67, and 68, 69, 70 shown in FIG. 6,different frequencies are effectively launched from different parts ofthe transducer; this is achieved by varying the interfinger spacingalong the transducer. Thus, the low and high frequency ends of the inputtransducer 67 may be left and right ends respectively as seen in thedrawing, while the low and high frequency ends of the output transducers68, 69, 70 are also the left and right ends respectively. In thesecircumstances the dispersion characteristics of the input 67 and output68, 69, 70 transducers tend to cancel each other out. It is arrangedthat the input transducer 67 and one output transducer, e.g., 68, haveidentical dispersion characteristics so that this delay line as a wholeis non-dispersive and the mode spacing is 1/γ_(a) at all frequencies(where γ_(a) is the delay time which in these circumstances isindependent of frequency). However, the dispersion characteristics of 69and 70 are successively mismatched from the input transducer 67 in sucha manner that for each delay line the intermode spacing isΔf.tbd.1/γ_(a) at all frequencies.

An additional advantage of using dispersive transducers is in theirrelative ease of matching to a 50Ω source for minimum insertion lossover large bandwidths. For this reason there may be occasions where fivetransducers of identical dispersion characteristics are used in thearrangement of FIG. 1.

The arrangement shown in FIG. 3 may also be modified to use dispersivetransducers in a manner similar to that described with reference to FIG.1.

FIG. 5 shows a bulk acoustic wave (BAW) delay line 45 for use in thefeedback loop of an amplifier to replace the delay line 15 of FIG. 3. Itcomprises an isopaustic glass block 46 with two polished flat andparallel ends 47, 48. One end 47 carries an input transducer 49 and theother end carries two output transducers 50, 51. Each transducer 49, 50,51 comprises a quartz crystal 52, 53, 54 sandwiched between two goldsheet electrodes 55, 56, 57, 58, 59, 60. Signals from output transducers50, 51 will be in phase; a 90° phase shift between outputs may beachieved by a λ_(o) /2 length of electromagnetic delay cable.Alternatively series/parallel tuning inductances may be added toone/both transducers so as to produce a phase difference betweenoutputs.

The BAW delay line is used in the same manner as the SAW line of FIG. 3,namely to change the frequency of oscillation, output to the amplifieris changed in phase steps of 90°, by using the output transducers 50, 51in turn and by reversing earth and live electrodes e.g. with biassed PINdiodes.

One problem with all the above oscillators is knowing the precisefrequency mode the device is operating at out of the many permittedmodes, e.g. after initial switch on. There are many possible solutionsto this problem, four of these are as follows.

One solution is to ensure that one mode is more strongly favoured thanall the others (e.g. the central mode in FIG. 2) and to switch theamplifier on slowly so that its gain gradually increases over a periodof say 1 second. It is known that the build-up time of oscillation isabout twenty times the acoustic delay time (typically 20 times 3μsec =60 μsec) giving adequate time for the central lobe to build-up beforethe other modes are allowed.

A second solution could be as follows. Allow the device to come on at anarbitrary frequency, and then to wind the frequency up through at leastthe full range. By correct design it can be made to stay at the highestfrequency when an attempt is made to wind it above the highest frequencyi.e. rather than jump to an arbitrary frequency. Thus this procedureenables one to attain the highest allowed frequency, and all subsequentchanges are referred to this (highest) frequency. Equally, the frequencycould be wound down to the lowest allowed value.

A third solution is to use a SAW oscillator having a SAW delay line onthe same substrate but with such oscillator having strong mode selectioni.e. it operates at one frequency only. One such oscillator havingstrong mode selection is described in U.K. patent application 7880/73U.S.A. patent applicaton Ser. No. 442,624 (now U.S. Pat. No. 3,950,713),and comprises (in a simple form) an input transducer whose effectivelength is equal to the centre to centre distance between input andoutput transducers. When using two delay lines on the same substrate thedelay line having strong mode selection is switched on and allowed tostabilise its frequency. When the oscillator having many possible modesis then started it will oscillate at the frequency already presenteither because of the acoustic waves on the substrate travelling on bothdelay line tracks or because of electrical interconnections common toboth oscillators.

A fourth solution is to incorporate a frequency discriminator into theoscillator, e.g. as described in U.K. patent application 52,237/73,U.S.A. Pat. No. 3,921,093. The voltage output from this discriminator(e.g. one or two SAW transducers) is a measure of the frequency ofoscillation. A new frequency may be selected by push-buttons whichgenerate a voltage equal to the discriminator output voltage at thedesired frequency. Any difference between this push-button voltage andthe discriminator output voltage causes the frequency changing mechanismto operate, until the difference is reduced to a negligible value.

What I claim is:
 1. A multi moded acoustic wave oscillator comprising anamplifier and a feedback loop connecting between the amplifier input andoutput and incorporating an acoustic delay line, said delay linecomprising a substrate able to support acoustic waves, an interdigitalfinger comb input transducer for launching acoustic waves along a trackin the substrate, a plurality of interdigital finger comb outputtransducers arranged on one side of the input transducer with eachoutput transducer across a different part of the track at a differentdistance from the input transducer and each being capable of supportingoscillation at a number of harmonic frequencies, said difference indistance being less than one half wavelength at the transducer centrefrequency, and switching means for cyclically connecting each outputtransducer in turn to the amplifier so that by sequential operation ofsaid switching means, the oscillator may be caused to oscillate at anyone of said harmonic frequencies, such number being much greate than thenumber of output transducers.
 2. An oscillator according to claim 1wherein the switching means comprises a plurality of solid stateelectrically controlled switches.
 3. An oscillator according to claim 1and comprising two output transducers spaced a quarter of the centrefrequency wavelength apart on the track with both combs in bothtransducers connected to the amplifier through the switching means. 4.An oscillator according to claim 1 and comprising four outputtransducers spaced along the track so that their outputs to theamplifier are spaced apart in phase by about 90 /2.
 5. An oscillatoraccording to claim 1 and comprising three output transducer spaced athird of the centre frequency wavelength apart along the track.
 6. Anoscillator according to claim 1 wherein the transducers arenon-dispersive.
 7. An oscillator according to claim 1 wherein at leastone of the transducers is dispersive.
 8. An oscillator according toclaim 1 wherein the length of the fingers within at least one of thetransducers varies along the length of the transducer.
 9. A multi modedacoustic wave oscillator comprising an amplifier, and a feedback loopconnecting between the amplifier input and output, and incorporating anacoustic delay line, said delay line comprising a substrate able tosupport acoustic waves, transducer means for launching acoustic wavesalong a track in the substrate, transducer means for receiving acousticwaves from the track, one of the transducer means comprising a singledispersive interdigital finger comb transducer while the othertransducer means comprises a plurality of dispersive interdigital fingercomb transducers each arranged across a different part of the track at adifferent distance from said single transducer and each being capable ofsupporting oscillation at a number of harmonic frequencies, saiddifference in distance being less than one wavelength at thetransducers' centre frequency, the dispersion characteristics of thesingle transducer and at least one of the plurality of transducers beingmatched so that the delay line formed by the two matched transducers isnon-dispersive and the frequency difference between frequency modes issubstantially constant, and switching means for connecting each one ofthe plurality of transducers in turn to the amplifier, so that theoscillator may be caused to oscillate at any one of said harmonicfrequencies, such number being much greater than the number of saidplurality of transducers.